Dual band antennas for wireless voice and data communications are known. For example, common frequency bands for GSM services include GSM900 and GSM1800.  GSM900 operates at 880 -960 MHz. Hereinafter, this set of frequencies will be referred to as the “Band 1”. GSM1800 operates in the frequency range of 1710 -1880 MHZ. Hereinafter, this set of frequencies will be referred to as the “Band 2”.
Antennas for communications in these bands of frequencies typically include an array of radiating elements connected by a feed network. For efficient transmission and reception of Radio Frequency (RF) signals, the dimensions of radiating elements are typically matched to the wavelength of the intended band of operation. Because the wavelength of the 900 MHz band is longer than the wavelength of the 1800 MHz band, the radiating elements for one band are typically not used for the other band. In this regard, dual band antennas have been developed which include different radiating elements for the two bands. See, for example, U.S. Pat. No. 6,295,028, U.S. Pat. No. 6,333,720, U.S. Pat. No. 7,238,101 and U.S. Pat. No. 7,405,710 the disclosures of which are incorporated by reference.
In these known dual band antennas, the radiating elements of the Band 2 may be interspersed with radiating elements of the Band 1, or nested within the radiating elements of the 900 MHz band, or a combination of nesting and interspersing. See, e.g., U.S. Pat. 7,283,101, FIG. 12; U.S. Pat. No. 7,405,710, FIG. 1, FIG. 7. In these known dual-band antennas, the radiating elements are typically aligned along a single axis. This is done to minimize any increase in the width of the antenna when going from a single band to a dual band antenna.
An increase in antenna width may have several undesirable drawbacks. For example, a wider antenna may not fit in an existing location or, if it may physically be mounted to an existing tower, the tower may not have been designed to accommodate the extra wind loading of a wider antenna. The replacement of a tower structure is an expense that cellular communications network operators would prefer to avoid when upgrading from a single band antenna to a dual band antenna. Also, zoning regulations can prevent of using bigger antennas in some areas.
Known dual band antennas, while useful, are not sufficient to accommodate future traffic demands. Wireless data traffic is growing dramatically in various global markets. There are growing number of data service subscribers and increased traffic per subscriber. This is due, at least in part, to the growing popularity of “smart phones,” such as the iPhone, Android-based devices, and wireless modems. The increasing demand of wireless data is exceeding the capacity of the traditional two-band wireless communications networks.
To address this increasing demand, wireless network operators are adding new wireless bands of frequencies. For example, the UMTS band operates at 1920 -2170  MHz. This set of frequencies is sufficiently close to the GSM1800 band that UMTS may be considered part of Band 2. Also, Digital Dividend spectrum includes 790 -862  MHz and will be considered hereinafter as part of Band 1. However, additional bands are being added. For example, LTE2.6 operates at 2.5 -2.7 GHz (Hereinafter “Band 3”) and WiMax operates at 3.4 -3.8 GHz (hereinafter “Band 4”). To make use of Bands 3 and 4 , wireless communications operators typically replace existing base station antennas with new multiband antennas.
However, simply adding additional cross-polarized radiating elements for Band 3 and Band 4 to a conventional dual band antenna poses certain difficulties. There is limited area for the inclusion of additional radiating elements, because the space between radiating elements of one band is already occupied by radiating elements of another band. Also, the Band 3 and Band 4 elements may introduce undesirable interference and distortion in the operation of the Band 1 and Band 2 elements.